The present invention relates to a charge transfer type transversal filter with programmable weighting coefficients.
As already known, a charge transfer device (CTD) such as a charge-coupled device (CCD) and a bucket-brigade device (BBD) is applicable to a transversal filter by taking advantage of its delay function.
If the weighting coefficients of multipliers included in the transversal filter are properly set, it is possible to form a filter with a desired frequency characteristic. In one of the conventional transversal filters, the weighting coefficients of the multipliers are set by control voltages externally applied to an integrated circuit. In this type conventional transversal filter, a plurality of weighting coefficient control voltage sources are provided outside the integrated circuit. For this reason, the number of terminals of the integrated circuit increases to make the device large in size. Further, the control voltages must be adjusted in their magnitudes independently.
In another example of a conventional transversal filter, a CCD analog shift register or capacitor memory is provided within an integrated circuit so as to store the weighting coefficient control voltages to be applied to the multipliers. In this type system, the number of terminals of the integrated circuit decreases but the storing time for the control voltages is restricted within a range of several milliseconds to several hundreds of milliseconds, so that the control voltages must repeatedly be loaded into the memory system. This makes the transversal filter system very complicated. The storing time is influenced by the manufacturing process and by temperature variations. Accordingly, it is very difficult to set the weighting coefficients with high accuracy. As known, in the CCD or the capacitor memory, the storing time is reduced by about 1/2 for each 10.degree. C. rise in temperature.